1. Field of the Invention
This invention relates to a developing assembly, an image-forming apparatus and a process cartridge which make use of a developer-carrying member used when an electrostatic latent image formed on a latent-image-bearing member, such as an electrophotographic photosensitive member or an electrostatic recording dielectric, is developed to render it visible in electrophotography.
2. Related Background Art
A number of methods are conventionally known as methods for electrophotography. Copies are commonly obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner to form a visible image (toner image), transferring the toner image to a transfer medium, such as paper, as the occasion, demands, and thereafter fixing the toner image to the transfer medium by heat, pressure or heat and pressure.
In general, methods of developing such electrostatic latent images by the use of toners, are roughly classified into a method in which a two-component developer comprised of a blend of a toner and a carrier is used and a method in which what is called a one-component developer is used to develop the latent image with a toner alone, without using any carrier.
Electrophotography has reached a level which can be satisfactorily used as document copying means for the time being. However, with the advancement of computers, high-definition display devices and so forth, higher image quality and higher grade reproduced full-color images, have been achieved by various methods, such as digital image processing and the application of alternating electric fields at the time of development. It is also sought in the future to achieve much higher image quality and much higher grade reproduced full-color images.
Among the methods of developing electrostatic latent images by the use of toners, the two-component developer, comprised of a blend of a toner and a carrier, is commonly preferably used in full-color copying machines or printers required to afford high image quality. In this developing system, the carrier provides the toner with positive or negative charge in a suitable quantity by triboelectric charging, and also carries the toner on the particle surfaces by the action of an electrostatic attraction force produced by the triboelectric charging.
The developer having a toner and a carrier is applied onto a developing sleeve internally provided with a magnet, in a stated layer thickness by the aid of a developer-layer thickness-control member, and is, by the use of magnetic force, transported to a developing zone formed between an electrostatic-image-bearing member (photosensitive member) and the developing sleeve. Across the photosensitive member and the developing sleeve, a certain development bias is kept applied, and the electrostatic latent image on the photosensitive member is developed with the toner at the developing zone.
Carriers included in such two-component developers are commonly roughly classified into a conductive carrier as typified by iron powder and what is called an insulating carrier comprised of magnetic particles such as iron powder, nickel powder or ferrite powder whose particle surfaces have been coated with an insulating resin. Where an alternating electric field is applied in order to achieve higher image quality, the carrier may lower the latent-image potential if it has a low electrical resistance to make it impossible to obtain good developed images. Hence, it is necessary for the carrier to have an electrical resistance at a certain level or higher. Where carrier cores are conductive, it is preferable to use a coated carrier. Also, preferably used as core materials are ferrite and magnetic-fine-particle-dispersed resin particles, having a high electrical resistance to a certain degree.
In general, the iron powder has so high a magnetic force as to make the developer form a hard magnetic brush in the developing zone where the toner in the developer develops the latent image. Hence, sweep marks may appear on the images or coarse images may appear to make it difficult to obtain developed images with high image quality. Accordingly, ferrite or a magnetic-fine-particle-dispersed resin carrier is preferably used also in order to make the carrier have a low magnetic force to achieve higher image quality.
In addition, in the case of the magnetic-fine-particle-dispersed resin carrier, it has a smaller specific gravity than the iron powder or ferrite. Hence, it not only can have a smaller intensity of magnetization per unit volume, but also may give less shear to the toner, and hence it can achieve not only high image quality, but also high developer durability. Thus, the magnetic-fine-particle-dispersed resin particles, having a high electrical resistance to a certain degree, are preferably used as the carrier.
As the developer-carrying member used in the conventional two-component developing system as mentioned above, for example, a metal, an alloy or a metallic compound is shaped or molded into a cylinder, and its surface is roughed by electrolysis, blasting, filing or the like to have a stated surface roughness to provide the developer-carrying member with surface unevenness. Such members having been treated for improving developer-transport performance are used. Of these, a sleeve made of aluminum is preferably used as the developer-carrying member in view of workability and low cost.
However, where the metal, alloy or metallic compound having a good workability is used, the sleeve tends to be deformed at the time of blasting to tend to cause uneven images. Moreover, where copies are made on many sheets, the hills that comprise the uneven sleeve surface may wear as a result of its rubbing with the developer when the developer is applied on the developing sleeve in a stated layer thickness by the aid of the developing-layer thickness-control member. At the same time, any fine powder and external additives of the toner may little by little come to adhere electrostatically and thermally to the sleeve surface, so that any contamination may begin to occur especially in the valleys of the uneven sleeve surface. Hence, the developing sleeve may come to have a materially smaller surface roughness, resulting in a lowering of the developer-transport performance.
In the case of a sleeve produced using a material comprised of stainless steel with superior wear resistance and provided with unevenness at its surface, too, the contamination of the sleeve surface due to the toner's fine powder and external additives as a result of many-sheet copying has not been able to be reduced to such an extent that the lowering of the developer-transport performance can be prevented in a satisfactory state. Because of the lowering of developer-transport performance due to such sleeve contamination, it has been difficult in the conventional developing sleeve to apply the developer stably onto the developing sleeve in a desired mass per unit area (hereinafter referred to as “M/S” which herein stands for mass on sleeve) However, the M/S, the quantity of developer on the developing sleeve, is closely connected to image quality by affecting density. Accordingly, it is important to attain the desired M/S stably over a long period of time, in order to long maintain a high-grade image quality.
In recent years, toners whose particle shape has been made substantially spherical have been brought forth for the purpose of further improving transfer performance and further improving image quality. Such toners, by virtue of their shape factors, have good releasability from photosensitive drums, so that a high transfer efficiency can be achieved. In particular, high-density large-area images can have a high grade. On the other hand, where two-component development is performed using such a toner, the coefficient of friction (between the sleeve and the developer may decrease because the particle shape of the toner is close to spherical. Thus, the conventional developing sleeve has an insufficient developer-transport performance. Moreover, in such a spherical toner, any external additives of the toner tend to be liberated, and hence any liberated external additives or any toner particles, in which the attached external additives are less, are liable to contaminate the sleeve surface. Thus, in the conventional developing sleeve, it has been more difficult to apply the developer in a stable M/S onto the developing sleeve.
With regard to the contamination of developing sleeve due to toner as stated above, the toner contamination and wear of the sleeve surface can be reduced to a certain extent when the magnetic-fine-particle-dispersed resin carrier is used, because the carrier can be made to have a low specific gravity and the carrier particles can be made spherical with ease to have less strain due to their carrier-particle shape, and hence the carrier may apply less load on the developer and the developer-carrying member when the developer layer thickness on the developing sleeve is controlled by means of the developer-layer thickness-control member. However, such contamination has not been reduced to such an extent that deterioration in the developer-transport performance can be prevented in a satisfactory state. Accordingly, it is sought to make further improvement.
As for the developer-carrying member, a method is disclosed in Japanese Patent Application Laid-open No. 01-277265 in which a rosin coat layer made of a triboelectrically chargeable resin having a conductive material or solid lubricant, such as carbon black or graphite dispersed therein, is formed on the developer-carrying member substrate in order to prevent any toner contamination that may be caused by electrostatic firm adhesion of the developer.
Where such a sleeve provided with the resin-coat layer having carbon black or graphite added thereto is used in a two-component developer developing assembly, the contamination of sleeve surface due to toner can be reduced. However, the resin coat layer on the developing sleeve has such insufficient wear resistance that the surface roughness tends to decrease because of the wear of the resin coat layer surface in many-sheet image reproduction. This makes it difficult to maintain the developer-transport performance stably on the developing sleeve.